from datetime import datetime

class User:
    def __init__(self, username, password):
        self.username = username
        self.password = password
        self.borrowed_books = []

class Admin(User):
    def __init__(self, username, password):
        super().__init__(username, password)

class Book:
    def __init__(self, title, author, isbn, quantity):
        self.title = title
        self.author = author
        self.isbn = isbn
        self.quantity = quantity
        self.borrow_records = []

class BorrowRecord:
    def __init__(self, book, user):
        self.book = book
        self.user = user
        self.borrow_time = datetime.now()
        self.return_time = None 